Nairobi -- With foreign navies patrolling the Gulf of Aden, Somali pirates have sharpened their tactics, using previous ransoms to expand and favourable seas to wreak havoc in the Indian Ocean.

With six hijackings in the space of four days, Somalia's pirates have dashed any hope that increasing naval presence in the region could significantly dent a scourge that is disrupting one of the world's busiest maritime trade routes.

Since the start of the year, piracy watchdogs had recorded a slump in the number of attacks and their success rate compared to 2008, during which pirates attacked close to 150 ships and harvested a bumper crop of ransom money.

Hans Tino Hansen, managing director of Denmark-based Risk Intelligence, said one of the main reasons for the sudden surge in attacks was simply the weather.

"Weather is king. Due to the profile of the pirates’ skiffs and other vessels, they are very dependent on favourable weather conditions, which has been the case east of Somalia lately," he told AFP.

This has allowed pirates to venture further out at sea and avoid the now heavily-patrolled shipping corridors in the Gulf of Aden, where maritime traffic bottlenecks in and out of the Red Sea.

"As with all organised crime, when law enforcement or the military starts to put pressure on them, they change in either 'business area' or geographical area and here it is the latter," Hansen explained.

Navy ships with an arsenal of sea and aerial assets under the command of the US navy, NATO, the European Union or single states have converged on the Gulf of Aden to secure aid deliveries and escort merchant vessels.

Even in the Gulf of Aden, four ships regularly spaced out can only control a quarter of the main corridor and four percent of the entire area.

This leaves the much broader eastern flank of Somalia wide open, prompting the US navy to reiterate its warning that the nearest navy ships might always be days away.

"The area involved off the coast of Somalia and Kenya as well as the Gulf of Aden equals more than 1.1 million square miles (2.5 million square kilometres), roughly four times the size of Texas or the size of the Mediterranean and Red Seas combined," it said in a recent statement.

Since April 4, Somali pirates have hijacked a US container ship, a small French sailing yacht, a British-owned Italian-operated cargo, a German container carrier, a Taiwanese fishing vessel and a Yemeni tugboat.

Several in the latest spate of attacks have come from the pirate group based on the eastern Somali coast, in Hobyo and Harardhere.

They scored some of the pirates' most spectacular successes late last year by seizing a Ukrainian cargo loaded with combat tanks and other weaponry, as well as a Saudi super-tanker carrying 100 million dollars in crude oil.

The combined ransoms paid for the release of these two ships alone is believed to fetch around eight million dollars and the pirates are known to significantly reinvest in better equipment.

LLoyd's Register-Fairplay analyst Jim Wilson said this week's attacks demonstrate that the pirates have improved their capacity.

"This latest bout of piracy is different from the previous attacks in that they have targeted container ships," he said in a statement.

"It has long been thought that because of their speed and high freeboard, the distance between the waterline and the deck, boxships are less vulnerable to pirate attack," Wilson explained.

Another factor is that some Somali clans that had previously not been involved in piracy are now joining the gold rush.

Lawless Somalia has not had a functioning central authority in close to two decades and piracy has emerged as one of the only viable businesses.

AFP/Expatica